In order to collect natural gas or crude oil from underground, a pit is dug toward a natural gas field or an oil field existing at several thousand meters from the ground surface to underground, and it is necessary to install a large transport pipe to the pit. In the transport pipe, a plurality of long steel pipes (so-called oil-well pipes) are connected to each other in a line. In recent years, in the viewpoint of productivity improvement, a need for a screw joint (so-called integral joint) for a steel pipe capable of directly connecting the oil-well pipes without using a coupling is increasing. The oil-well pipe having a male screw formed on an outer circumferential surface of one pipe end and a female screw formed on an inner circumferential surface of the other pipe end is used as the integral joint. That is, the integral joint includes the male screw (pin) which is spirally carved on the outer circumferential surface of one pipe end of the oil-well pipe, and the female screw (box) which is spirally carved on the inner circumferential surface of one pipe end of the other oil-well pipe connected to the oil-well pipe.
Conventionally, when the oil-well pipes are secured to each other, in order to prevent seizure of the joint portion, lubricating oil (API dope) including heavy metals such as Pb is applied to at least one of the male screw and the female screw of the oil-well pipe. On the other hand, in a region in which use of the API dope is limited under a severe environmental regulation, environment protective lubricating oil (green dope) not including heavy metals may be used. Since lubricity of the green dope is worse than that of the API dope, the seizure easily occurs in the joint portion. Thereby, when the green dope is used as the lubricating oil, in order to compensate for lack of the lubricity of the green dope and prevent occurrence of the seizure, it is preferable that an electro plating layer such as copper be formed on at least one surface of the male screw and the female screw carved on the pipe end of the oil-well pipe.
For example, in Patent Document 1 below, a device is disclosed which forms an electro plating layer on a surface of a male screw (pin) carved on one pipe end of the oil-well pipe, that is, on an outer circumferential surface of one pipe end of the oil-well pipe.